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Conclusions: These findings indicate that 1) maximal stroke volume is the critical determinant of the high V̇O2max in child cyclists and 2) factors that influence resting stroke volume are important in defining V̇O2max differences between child endurance athletes and untrained boys.

The cardiovascular characteristics of young male adult endurance athletes have been well-delineated. These athletes typically demonstrate a V̇O2max of 70–75 mL·kg−1·min−1 (approximately 50% greater than the nonathletic population), a manifestation principally of a greater maximal cardiac output (7). The higher cardiac reserve in athletes is associated with a greater resting left ventricular size compared with nonathletes (15,40). Consequently, factors such as resting bradycardia, enhanced plasma volume, and left ventricular enlargement have been suggested as potential determinants of the superior maximal cardiac output (and, consequently, V̇O2max) in highly trained endurance athletes. Alternatively, some studies have suggested that the stroke volume in a progressive exercise test steadily rises in endurance athletes compared to the plateau typically observed in nonathletes (8,26). This implies that factors such as enhanced diastolic filling or superior skeletal muscle pump function might also be important in establishing higher V̇O2max in athletes.

Limited information has suggested that qualitative differences may exist in the cardiac responses of child endurance athletes to exercise compared with those of their adult counterparts. Mean V̇O2max in studies of highly trained prepubertal male distance runners is typically 60–65 mL·kg−1·min−1, compared with an average value of 52 mL·kg−1·min−1 in nontrained boys (21). Although the lower V̇O2max levels in child compared with adult athletes could reflect duration of and/or intensity of training, it has been suggested that a “ceiling” may exist for improving V̇O2max with aerobic training before puberty (13).

Whether child endurance athletes manifest larger resting left ventricular size compared with nonathletes is controversial. The number of studies indicating similar ventricular diastolic diameters in young athletes and nonathletes is comparable to those indicating larger left ventricular dimensions in child athletes (10,16,19,20,25,32,35,38). No previous data are available to examine the relative roles of cardiac output and peripheral oxygen extraction (arteriovenous oxygen difference) in defining the greater V̇O2max in child athletes compared with nonathletes.

This study utilized two-dimensional echocardiography at rest and Doppler echocardiography with exercise to examine the cardiovascular characteristics of well-trained prepubertal child cyclists. Specifically, this investigation was designed to compare resting cardiac dimensions, the relative importance of Q̇max and maximal arteriovenous oxygen difference as determinants of V̇O2max, and the pattern of stroke volume response to exercise in child cyclists compared with a group of nontraining age-matched children. In addition, this study sought to compare characteristics of child cyclists with those previously described in adult endurance athletes.

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